In recent years, in order to reduce the amount of CO2 emissions, there are an increasing number of electrically driven vehicles, such as a hybrid vehicle having a motor and an engine mounted thereon and an electric vehicle configured to be driven only with a motor. Those electrically driven vehicles having the motors mounted thereon each include, in addition to the motor, an inverter for driving the motor, a battery serving as a power supply, and the like.
Further, in those electrically driven vehicles, in order to improve the output of the motor, developments have been made to use a high battery voltage and to increase the battery voltage with a boost converter. In addition, in those electrically driven vehicles, development has been made to improve performance of a cooling function in order to prevent a temperature of the motor from being high even after long-term use.
However, when the motor generates drive power to run the vehicle, for example, if the motor is used in a high output state for a long time on a mountainous highway or the like, a temperature of a stator or a rotor rises to reach an upper limit value. In this case, a permanent magnet to be used for the rotor is demagnetized when its temperature exceeds the upper limit value, resulting in a problem in that motor torque is reduced.
For protection against this situation, countermeasures are taken to mount a temperature sensor such as a thermistor on the stator. On the other hand, it is difficult to mount a temperature sensor such as a thermistor on the permanent magnet to be used for the rotor because the permanent magnet is a rotating portion, resulting in a problem in that the temperature of the permanent magnet cannot be measured.
In order to solve such problems, there has been proposed a motor control device configured to estimate a temperature of a permanent magnet based on a temperature of a stator coil detected by stator temperature detection means, a fluid temperature of a coolant, a thermal resistance ratio as the ratio of a thermal resistance between the coolant and the stator coil and a thermal resistance between the stator coil and the permanent magnet, and a heat generation ratio (heat dissipation characteristics) between the stator coil and the permanent magnet (see, for example, Patent Literature 1).
Further, there has been proposed a motor temperature estimation device configured to estimate a temperature of a permanent magnet based on a basic formula of a motor by using a voltage applied to the motor, a current flowing through the motor, and a model formed of a resistance and an inductance that are motor constants (to estimate the temperature by using temperature characteristics of permanent magnet magnetic flux) (see, for example, Patent Literature 2).